In the fine machining of workpieces using a laser, in particular in the precision cutting of. stents, the process parameters must be matched to the respective material, particularly for critical materials such as nitinol, or alloys based on cobalt (Co) and chromium (Cr), in order to achieve an optimum process result. These process parameters on the one hand are determined by the characteristics of the laser beam itself, and on the other hand are specified by additional physical parameters independent of the laser beam, such as the type of shield gas, the gas pressure during laser machining, the feed rate, or the machining geometry, in particular the geometry of the cuts being made. The important parameters with respect to the laser beam are essentially the wavelength, the beam quality, the beam caustic, the average power and energy introduced into the workpiece, the shape and length of the pulse in lasers operating in pulse mode, and the repetition rate of the laser pulses.
In particular for precision cutting, the characteristics of the cutting edges, such as their shape, any burrs present and their removability, or roughness, are sensitive to influence from these process parameters, and in the manufacture of stents are of critical importance for the problem-free therapeutic use thereof.
For precision cutting of stents, Nd:YAG solid-state lasers have generally been used heretofore which operate in pulse mode and are optically pumped using flash lamps. For such a laser, however, the above-referenced process parameters with respect to the laser beam cannot always be optimized to the particular process, since these process parameters cannot be adjusted independently of one another. Thus, for pulse durations <100 μs, for example, which are necessary in particular for precision cutting, the pulse energy of a laser pulse is coupled with the peak pulse power and the pulse duration. A typical laser pulse is diagrammatically illustrated in FIG. 4, in which the power output of the laser is plotted versus time. It is seen from the figure that the laser pulse is composed of multiple individual pulses—so-called “spikes”—which, starting from an initial pulse, die out in an approximately exponential manner to a constant power level (CW power). This pulse shape is essentially independent of the energy introduced into the laser-active medium by the flash lamp. In this manner, for example, an increase in the pulse energy is always associated with an increase in the peak pulse power of the initial pulse, and/or with an extension of the pulse duration. In addition, for technical reasons the pulse frequency is limited to values below 3 kHz.
A laser cutter for manufacturing stents is known from US 2005/0035101 A1, in which a diode-pumped fiber laser is used to avoid the above-referenced problems. In this known laser cutter a function generator is used to actuate the diode laser used for pumping, which allows shaping of the pump laser pulse generated by the diode laser. In this manner, pulse shapes for the laser pulse generated by the fiber laser are produced which correspond to the pulse shape of the current pulse which is used for operating the diode laser and controlled by the function generator.